1. Field of the Invention
The present invention relates to a synchronous tracking device for a receiver for spread spectrum communication in a carrier-coherent system in which the clock of a pseudo noise code is equal to a quotient of the carrier frequency divided by an integer, or the clocks of the pseudo noise (PN) code and the carrier are obtained from a common frequency source.
2. Background of the Invention
FIG. 1 shows a conventional transmitter for spread spectrum communication in a carrier-coherent system. The output from a common frequency source, oscillator 30, is common to a modulator 35, a one-N-th frequency divider 31, and a one-M-th frequency divider 32. The outputs from the dividers 31 and 32 are respectively supplied to a pseudo noise generator 33 and a data generator 34. A signal modulated by the modulator 35 via Exclusive OR (XOR) gates 35a, 35b is transmitted as signal S.
FIG. 2 shows a conventional receiver including a Costas demodulator loop 200, which is appropriate to recover the received data from a corresponding received signal S'. This conventional Costas loop has a voltage-controlled oscillator (VCO) 14, a phase shifter 18, three multipliers 16, 17 and 22, low pass filters (LPF) 19 and 20, and a loop filter 21. The received signal S' is despread by a multiplier 1. The output of the multiplier 1 is applied to a band pass filter 4 to thereby cut off noise outside of the filter's pass band. The output of band pass filter 4 is supplied to the Costas loop 200 to recover received data. The Costas loop 200 includes VCO 14, the output of which is applied to a multiplier 24 via a frequency multiplying unit 23 to reduce the frequency of the received signal. The received signal frequency is reduced thereby to facilitate further signal processing.
The band pass filter 4 included in the conventional receiver shown in FIG. 2 is generally made of a narrow band pass filter to render the ratio of carrier to noise (C/N ratio) large. Accordingly, such a band pass filter 4 causes a delay in the Costas loop. If there is delay in the Costas loop, the loop's gain and frequency response are limited so that the loop's lock range and capture range cannot be widened. For that reason, even if the phase of the local pseudo noise code of the receiver temporarily coincides with that of the pseudo noise code of the received signal input to the receiver, capture or locking-in cannot be performed in some cases, making it impossible to recover the data transmitted.